Proton conducting membrane using a solid acid

ABSTRACT

A solid acid material is used as a proton conducting membrane in an electrochemical device. The solid acid material can be one of a plurality of different kinds of materials. A binder can be added, and that binder can be either a nonconducting or a conducting binder. Nonconducting binders can be, for example, a polymer or a glass. A conducting binder enables the device to be both proton conducting and electron conducting.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of and claimspriority to U.S. application Ser. No. 09/439,377, filed Nov. 15, 1999,which claims the benefit of U.S. provisional applications serial No.60/116,741, filed Jan. 22, 1999, serial No. 60/146,946, filed Aug. 2,1999, serial No. 60/146,943 filed Aug. 2, 1999, and serial No.60/151,811, filed Aug. 30, 1999.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

[0002] The invention described herein was made in the performance ofwork under a NASA contract and is subject to the provisions of PublicLaw 96-517 (U.C.C. 202) in which the Contractor has elected to retaintitle.

FIELD

[0003] The present application describes a proton conducting membraneformed using an solid acid in its solid phase. More specifically, thepresent application teaches a proton conducting membrane, formed usingan solid acid mixed with a supporting binder material, that isimpermeable to fluids such as gas and water, can operate withouthydration, and has high proton conductivity.

BACKGROUND

[0004] Proton conducting materials have a number of applications. Protonconducting membranes are widely utilized in devices which use a chemicalreaction to produce or store electricity, or use electricity to drive achemical process. Materials which conduct both protons and electrons(“mixed proton and electron conductors”) are utilized in relatedapplications.

[0005] Electrochemical devices depend on the flow of protons, or theflow of both protons and electrons through a proton conducting membrane.Exemplary electrochemical devices include a fuel cell, an electrolysiscell, a hydrogen separation cell, a battery, a supercapacitor, and amembrane reactor. There are other electrochemical devices which also usea proton conducting membrane.

[0006] An important use for proton conducting membranes is in fuelcells. Fuel cells are attractive alternatives to combustion engines forthe generation of electricity because of their higher efficiency and thelower level of pollutants they produce. A fuel cell generateselectricity from the electrochemical reaction of a fuel e.g. methane,methanol, gasoline, or hydrogen, with oxygen normally obtained from air.

[0007] There are three common types of fuel cells used at temperaturesclose to ambient. A direct hydrogen/air fuel cell system stores hydrogenand then delivers it to the fuel cell as needed.

[0008] In an indirect hydrogen/air fuel cell, hydrogen is generated onsite from a hydrocarbon fuel, cleaned it of carbon monoxide (CO), andsubsequently fed to the fuel cell.

[0009] A direct methanol fuel cell (“DMFC”), feeds methanol/watersolution directly to the fuel cell, e.g., without any fuel processing.One type of DMFC has been described, for example, in U.S. Pat. No.5,559,638. There are various advantages and disadvantages inherentwithin all three configurations. All are, to a greater or lesser extent,limited by the performance of the proton conducting membrane.

[0010] Nafion™, a perfluorinated sulphonic acid polymer, is often usedas a membrane material for fuel cells which operate at temperaturesclose to ambient. Other hydrated polymers have also been used as protonconductive materials. Membranes of modified perfluorinated sulfonic acidpolymers, polyhydrocarbon sulfonic acid polymers, and composites thereofare also known. These and related polymers are used in hydrated form.Proton transport occurs by the motion of hydronium ions, H₃O⁺. Water isnecessary in order to facilitate proton conduction. Loss of waterimmediately results in degradation of the conductivity. Moreover, thisdegradation is irreversible—a simple reintroduction of water to thesystem does not restore the conductivity. Thus, the electrolytemembranes of these hydrated polymer-based fuel cells must be kepthumidified during operation. This introduces a host of balance-of-plantneeds for water recirculation and temperature control.

[0011] A second limitation derives from the need to maintain water inthe membrane. In order to maintain hydration, the temperature ofoperation cannot exceed 100° C. without cell pressurization. Hightemperature operation could be desirable, however, to increase catalystefficiency in generating protons at the anode (in both H₂ and directmethanol fuel cells) and to improve catalyst tolerance to carbonmonoxide (“CO”). CO is often present in the fuel that is used in thefuel cells. The CO can poison the precious metal catalysts. This isparticularly problematic in indirect hydrogen/air fuel cells for whichhydrogen is generated on site. High temperatures also benefit thereduction reaction on the cathode.

[0012] Another limitation of hydrated polymer electrolytes occurs inapplications in methanol fuel cells. These polymers can be permeable tomethanol. Direct transport of the fuel (i.e. methanol) across themembrane to the air cathode results in losses in efficiency.

[0013] Alternate proton conducting materials, which do not requirehumidification, which can be operated at slightly elevated temperatures,and/or which are impermeable to methanol, are desirable for fuel cellapplications.

[0014] In the field of hydrogen separation, a proton conducting membraneis utilized to separate hydrogen from other gases such as CO and/or CO₂.Palladium is often used for this application. Palladium is permeable tomolecular hydrogen, but not in general to other gases. There aredrawbacks to the use of this material. It is expensive and the hydrogendiffusion rate is low. It would be desirable to develop new materialswhich are less expensive and exhibit higher proton/hydrogen transportrates.

[0015] In general, materials utilized in other electrochemical devicessuch as electrolysis cells, batteries, supercapacitors, etc., includeliquid acid electrolytes, which are highly corrosive, and solid polymerproton conductors, which require humidification or exhibit insufficientproton conductivity. High conductivity, high chemical and thermalstability solid membranes with good mechanical properties are desirablefor all of these applications.

SUMMARY

[0016] The present specification defines a new kind of material for aproton conducting membrane. Specifically, a proton conducting materialis formed using an solid acid. The solid acid can be of the general formM_(a)H_(b)(XO_(t))_(c) or M_(a)H_(b)(XO_(t))_(c).nH₂O,

[0017] where:

[0018] M is one or more of the species in the group consisting of Li,Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Tl and NH4⁺ or Cu⁺;

[0019] X is one or more of the species in the group consisting of Si, P,S, As, Se, Te, Cr and Mn; and

[0020] a, b, c, n and t are rational numbers.

[0021] Solid acids do not rely on the presence of hydronium ions forproton transport, thus they do not require hydration for use as protonconductors.

[0022] A preferred solid acid used according to this specification is asolid phase solid acid that exhibits a superprotonic phase, a phase inwhich the solid has disorder in its crystal structure and a very highproton conductivity.

[0023] An embodiment uses a structural binder or matrix material toenhance the mechanical integrity and/or chemical stability of themembrane. That structural binder can be many different kinds ofmaterials in the different embodiments. In particular, the structuralbinder can be a polymer, a ceramic, or an oxide glass.

[0024] Another embodiment uses an electronically conducting material asa matrix. This creates a membrane which conducts both protons andelectrons.

[0025] The resulting material can be used for a proton conductingmaterial in a device that relies on the flow of protons or the flow ofboth protons and electrons across a membrane, herein an“electrochemical” device e.g. a fuel cell, a hydrogen separationmembrane, or a electrolysis cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 shows an exemplary hydrogen/air fuel cell using an solidacid supported by a binder as its proton conducting membrane.

[0027]FIG. 2 shows an exemplary direct methanol fuel cell using an solidacid supported by a binder as its proton conducting membrane

[0028]FIG. 3 shows a hydrogen separation membrane for the removal of COand other gases from hydrogen;

[0029]FIG. 4 shows another type of hydrogen separation membrane made ofa proton conducting composite; and

[0030]FIGS. 5 and 6 show a membrane reactor.

DETAILED DESCRIPTION

[0031] The present application teaches using an solid acid as a protonconducting membrane.

[0032] A solid acid can be of the general formM_(a)H_(b)(XO_(t))_(c).nH₂O,

[0033] where:

[0034] M is one or more of the species in the group consisting of Li,Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Tl and NH₄ ⁺;

[0035] X is one or more of the species in the group consisting of Si, P,S, As, Se, Te, Cr and Mn; and

[0036] a, b, c, n and t are rational numbers; with t preferably being 3or 4, and where t≧0.

[0037] The solid acids used herein are compounds, such as CSHSO₄, whoseproperties are intermediate between those of a normal acid, such asH₂SO₄₁ and a normal salt, such as Cs₂SO₄. In general, the chemicalformula of the solid acids of the type used according to the presentspecification can be written as a combination of the salt and the acid.

[0038] In general, solid acids are comprised of oxyanions, for exampleSO₄, SO₃, SeO₄, SeO₃, SiO₄, PO₄ or AsO₄, etc., which are linked togethervia O—H . . . O hydrogen bonds. The structure may contain more than onetype of XO₄ or XO₃ group, and may also contain more than one type of Mspecies.

[0039] Certain solid acids are solid materials at room temperature.

[0040] Many different solid acids are contemplated by thisspecification. One example of a material that can be used as the solidacid is CsHSO₄, which is intermediate between Cs₂SO₄ (a normal salt) andH₂SO₄ (a normal acid). In this case, the solid acid can be written as0.5 Cs₂SO₄*0.5H₂SO₄. Another example, using the same salt and the sameacid, is 1.5 Cs₂SO₄*0.5H₂SO₄, to give Cs₃H(SO₄)₂.

[0041] Other examples are:

[0042] CsH₂PO₄, Cs₅(HSO₄)₃(H₂PO₄)₂, Cs₂(HSO₄)(H₂PO₄), Cs₃(HSO₄)₂(H₂PO₄),Cs₃(HSO₄)₂(H_(1.5)(S_(0.5)P_(0.5))O₄), Cs₃H₃(SO₄)₄.xH₂O, TlHSO₄,CsHSeO₄, Cs₂(HSeO₄)(H₂PO₄), Cs₃H (SeO₄)₂(NH₄)₃H(SO₄)₂,(NH₄)₂(HSO₄)(H₂PO₄), Rb₃H(SO₄)₂, Rb₃H(SeO₄)₂, Cs_(1.5)Li_(1.5)H(SO₄)₂,Cs₂Na(HSO₄)₃, TlH₃(SeO₃)₂, CsH₂AsO₄(NH₄)₂(HSO₄)(H₂AsO₄), CaNaHSiO₄

[0043] The preferred material for any specific electrochemical devicedepends on the application. For example, Cs₂(HSO₄)(H₂PO₄) may bepreferred for electrochemical devices where high conductivity iscritical. (NH₄)₃H(SO₄)₂ may be preferred where low cost is critical.CaNaHSiO₄ may be preferred where chemical stability is critical.

[0044] Solid acids have certain characteristics that can be advantageouswhen used as a proton conducting membrane. The proton transport processdoes not rely on the motion of hydronium ions, thus solid acids need notbe humidified and their conductivity is substantially independent ofhumidity. Another advantage is that solid acids are generally stableagainst thermal decomposition at elevated temperatures. The thermaldecomposition temperature for some of the solid acids described in thisspecification, e.g., CaNaHSiO₄, can be as high as 350° C. Since solidacids need not be humidified, solid acid based membranes can be operatedat elevated temperatures, e.g. temperatures above 100° C.

[0045] The conductivity of solid acids may be made purely protonic, orboth electronic and protonic depending on the choice of the X element inthe chemical formula M_(a)H_(b)(XO₄)_(c).nH₂O orM_(a)H_(b)(XO₃)_(c).nH₂O. That is, by using a given amount of a variablevalence element such as Cr or Mn for X, the solid acid can be made toconduct electrons as well as protons.

[0046] Another advantage is caused by the structure of the solid acidsthemselves. Since solid acids are dense, inorganic materials, they areimpermeable to gases and other fluids that may be present in theelectrochemical environment, e.g., gases and hydrocarbon liquids.

[0047] The materials are also relatively inexpensive.

[0048] This combination of properties: good conductivity in dryenvironments, conductivity which can be controlled to be either purelyproton conducting or both electron and proton conducting, impermeabilityto gases and hydrocarbon liquids, serviceability at elevatedtemperatures, e.g. temperatures over 100° C. and relatively low cost,render solid acids as useful materials for use as membranes inelectrochemical devices.

[0049] Solid acids exhibit another advantageous property forapplications in proton conducting membranes. Under certain conditions oftemperature and pressure, the crystal structure of a solid acid canbecome disordered. Concomitant with this disorder is an highconductivity, as high as 10⁻³ to 10⁻² Ω⁻¹ cm⁻¹. Because of the highproton conductivity of the structurally disordered state, it is known asa superprotonic phase. The proton transport is believed to befacilitated by rapid XO₄ or XO₃ group reorientations, which occurbecause of the disorder.

[0050] Many solid acids enter a superprotonic state at a temperaturebetween 50 and 150° C. at ambient pressures. The transition into thesuperprotonic phase may be either sharp or gradual. The superprotonicphase is marked by an increase in conductivity, often by several ordersof magnitude. At temperatures above the transition temperature, thesolid acid is superprotonic and retains its high proton conductivityuntil the decomposition or melting temperature is reached.

[0051] Solid acids that undergo a superprotonic transition include:

[0052] CsHSO₄, Cs₂(HSO₄)(H₂PO₄), Cs₃(HSO₄)₂(H₂PO₄),Cs₃(HSO₄)₂(H_(1.5)(S_(1.5)P_(0.5))O₄), Cs₅H₃(SO₄)₄.xH₂O, CsHSeO₄,Cs₃H(SeO₄)₂, (NH₄)₃H(SO₄)₂, Rb₃H(SeO₄)₂.

[0053] The superprotonic phases of solid acids have increasedconductivity. An interesting embodiment is a solid acid operated at atemperature above the superprotonic transition temperature, and belowthe decomposition or melt temperature.

[0054] Despite the many advantageous properties of solid acids, problemscan be encountered in trying to implement them in electrochemicaldevices because many solid acids are water soluble. They can also bedifficult to process into large area membranes, and they often have poormechanical properties. Some solid acids, such as CaNaHSiO₄ and othersilicates, are not soluble in water.

[0055] Because of these difficulties, a disclosed embodiment includes acomposite comprised of an solid acid embedded in a supporting matrix.The solid acid part of the composite provides the desiredelectrochemical activity, whereas the matrix provides mechanical supportand also may increase chemical stability. Different materials arecontemplated herein for use as the supporting matrix.

[0056] In light of the properties of solid acids outlined above, thepreferred embodiment is a composite material comprised of a solid acidembedded in a supporting matrix and operated at a slightly elevatedtemperature. In such a composite, the solid acid is in its superprotonicphase, exhibits high conductivity, and provides the desiredelectrochemical functions; the support matrix may provide mechanicalsupport, and it may also serve to protect the solid acid from water inthe environment. A high temperature of operation can render the solidacid into its superprotonic state. A high temperature of operation canalso ensure that any water present in the electrochemical device will bepresent in the form of steam rather than liquid water, making the H₂Oless likely to attack the solid acid.

[0057] Hydrogen/Air Fuel Cells

[0058] A hydrogen/air fuel cell is shown in FIG. 1, in which the protonconducting membrane is a solid acid/matrix composite of the typedescribed herein. Because the membrane need not be humidified, the fuelcell system can be simpler than one which uses a hydrated polymermembrane. The humidification system normally required for fuel cellutilizing a Nafion or related polymer membrane can be eliminated inFIG. 1. Hence, less rigid temperature monitoring and control may be usedin the solid acid based system as compared with Nafion based fuel cellsystems. These differences allow a less-costly fuel cell system.

[0059] Because the membrane need not be humidified, the fuel cell shownin FIG. 1 can be operated at temperatures above 100° C. The tolerance ofthe Pt/Ru catalysts to carbon monoxide CO poisoning increases withincreasing temperature. Thus, a fuel cell such as shown in FIG. 1,operated at a temperature above 100° C. may withstand higherconcentrations of CO in the hydrogen fuel than a Nafion based fuel cellwhich is typically operated at a temperature lower than 100° C.

[0060] The high temperature of operation also enhances the kinetics ofthe electrochemical reactions, and can thereby result in a fuel cellwith higher overall efficiency than one based on Nafion or otherhydrated polymers.

[0061] Direct Methanol Fuel Cells

[0062] A direct methanol fuel cell is shown in FIG. 2. The protonconducting membrane is a solid acid/matrix composite of the typedescribed herein. Because the membrane need not be humidified, the fuelcell system is much simpler and thus less costly than state of the artdirect methanol fuel cell systems. The humidification system normallyrequired for fuel cell utilizing a Nafion or related polymer membrane iseliminated in FIG. 2. Furthermore, temperature monitoring and control inthe solid acid based system does not need to be as tight as in Nafionbased fuel cell systems. Because the solid acid based membrane need notbe humidified, the fuel cell may be operated at elevated temperatures.High temperatures can enhance the kinetics of the electrochemicalreactions. This can result in a fuel cell with very high efficiency.

[0063] Another significant advantage of the fuel cell shown in FIG. 2over state of the art direct methanol fuel cells results from thedecreased permeability of the membrane to methanol. In state of the artdirect methanol fuel cells, in which Nafion or another hydrated polymerserves as the membrane, methanol cross-over through the polymericmembrane lowers fuel cell efficiencies. The impermeability of a solidacid membrane can improve this efficiency.

[0064] Hydrogen Separation Membranes

[0065] The Ru/Pt catalyst in a hydrogen/air fuel cell is sensitive to COpoisoning, particularly at temperatures close to ambient. Therefore, inan indirect hydrogen/air fuel cell, the hydrogen produced by thereformer is often cleaned, of e.g. CO to below 50 ppm, before it entersthe fuel cell for electrochemical reaction.

[0066] In FIG. 3, a hydrogen separation membrane is shown for theremoval of CO and other gases from hydrogen. The hydrogen separationmembrane is made of a mixed proton and electron conducting membrane, asdescribed herein. Hydrogen gas, mixed with other undesirable gases, isintroduced onto one side of the membrane. Clean hydrogen gas isextracted from the other side of the membrane.

[0067] On the inlet side of the membrane, hydrogen gas is dissociatedinto H+ and e−. Because the membrane is both proton conducting andelectron conducting, both of these species can migrate through themembrane. However, the membrane is substantially impermeable to othergases and fluids. Hence, CO and other undesirable gases or fluids cannotso migrate. On the outlet side of the membrane, the H+ and e− recombineto form hydrogen gas. The overall process is driven by the hydrogenchemical potential gradient, which is high on the inlet side of themembrane and low on the outlet side of the membrane.

[0068] Another type of hydrogen separation membrane is shown in FIG. 4.The membrane is made of a proton conducting composite of the typedescribed herein, and is connected to a current source. Hydrogen gas,mixed with other undesirable gases, is introduced onto one side of themembrane and clean hydrogen gas is extracted from the other side of themembrane. Application of a current causes the hydrogen gas to dissociateinto H+ and e−. Because the membrane conducts only protons, theseprotons are the only species which can migrate through the membrane. Theelectrons migrate through the current source to the outlet side of themembrane, where the H+ and e− recombine to form hydrogen gas. Themembrane is substantially impervious to other gases and fluids. Hence,CO and other undesirable gases or fluids cannot migrate through theproton conducting membrane. The overall process is driven by electriccurrent applied via the current source.

[0069] Membrane Reactors

[0070] In FIG. 5 a membrane reactor is shown, in which a mixed protonand electron conducting membrane of the type described herein isutilized. The general reaction is that reactants A+B react to formproducts C+D, where D is hydrogen gas. Use of a 5 mixed proton andelectron conducting membrane in this reactor can enhance the reaction togive yields that exceed thermodynamic equilibrium values. On the inletside of the membrane reactor, the reactants form products C+H2. Underequilibrium conditions, the hydrogen concentration builds up and theforward reaction is slowed. With the use of the mixed hydrogen andelectron conducting membrane, the hydrogen is immediately extracted fromthe reaction region via transport through the membrane, and the forwardreaction is enhanced. Examples of reactions in which yield could beenhanced by using such a membrane reactor include (1) the steamreformation of methane (natural gas) to produce syngas: CH4+H2O→CO+3H2;(2) the steam reformation of CO to produce CO2 and H2: CO+H2O→CO2+H2;(3) the decomposition of H2S to H2 and S, (4) the decomposition of NH3to H2 and N2; (4) the dehydrogenation of propane to polypropylene; and(5) the dehydrogenation of alkanes and aromatic compounds to variousproducts.

[0071] In FIG. 6 a second type of membrane reaction is shown, again,utilizing a mixed proton and electron conducting membrane of the typedescribed herein. In this case, the general reaction is that thereactants A+B form the products C+D, where B is hydrogen. The hydrogenenters the reaction region via transport through the mixed conductingmembrane, whereas the reactant A is introduced at the inlet to themembrane reactor, and is mixed with other species. The manner in whichthe hydrogen is introduced into the reactant stream (through themembrane) ensures that only the reactant A, and none of the otherspecies reacts with hydrogen. This effect is termed selectivehydrogenation.

[0072] The mixed proton and electron conducting membranes describedherein provide an advantage over state-of-the-art membranes in that theconductivity is high at temperatures as low as 100° C., and themembranes are relatively inexpensive. Selective hydrogenation attemperatures close to ambient may have particular application insynthesis of pharmaceutically important compounds which cannot withstandhigh temperatures.

[0073] According to a first class of materials, the solid acid is mixedwith a supporting structure that is electrochemically unreactive, toform a composite. A first embodiment uses a solid acid mixed with amelt-processable polymer as the supporting matrix structure.

[0074] The solid acid (CHS) was prepared from aqueous solutionscontaining stoichiometric amounts of Cs₂CO₃ and H₂SO₄. CrystallineCsHSO₄ and a small amount (˜8 wt %) of the related compoundCs₅H₃(SO₄)₄.xH₂O (which also exhibits superprotonic behavior) wereobtained upon introduction of methanol into the solution. Compositemembranes of the solid acid and poly(vinylidene fluoride) were preparedby simple melt-processing methods. The two components were lightlyground together then hot-pressed at 180° C. and 10 kpsi for 15 minutes.Volume ratios of CHS:PVDF from 100% CsHSO₄ to 100% PVDF were prepared in10 vol % increments.

[0075] Another example of a composite contains a solid acid and athermoset polymer, which can be mixed in with the solid acid in monomeror prepolymer form, and then polymerized in situ.

[0076] The solid acid (CHS) was prepared from aqueous solutionscontaining stoichiometric amounts of Cs₂CO₃ and H₂SO₄. CrystallineCsHSO₄ and a small amount (˜8 wt %) of the related compoundCs₅H₃(SO₄)₄.xH₂O (which also exhibits superprotonic behavior) wereobtained upon introduction of methanol into the solution. Compositemembranes of the solid acid and the polyester resin marketed under thename Castoglas by Buehler, Inc. were synthesized simply by lightlygrinding the solid acid and pre-polymer together and then adding thepolymerization/crosslinking catalyst. A material with a 50:50 volumeratio was prepared.

[0077] Another example of a thermoset polymer—solid acid compositecomprises the solid acid (NH₃)₃H(SO₄)₂ and the polymerpoly(dicyclopentadiene) or poly DCPD.

[0078] The solid acid, TAHS, was prepared from aqueous solutions of(NH₄)₂SO₄ and H₂SO₄. The solid acid was ground then mixed with themonomer dicyclopentadiene. The polymerization catalyst was introducedinto the mixture, which was then poured onto a Teflon plate and pressedinto a thin film. The film was cured at 100° C. for approximately 2hours. Materials with 25 and 17 vol % TAHS were prepared.

[0079] Another method for preparing solid acid/polymer composites issuspension coasting. For this, CsHSO₄ was dissolved in a water/ethanolsolution. The polymer PVDF was then dispersed into this solution. Acomposite membrane was formed by casting the suspension and allowing thesolvents to evaporate. Composite membranes comprised of a solid acid anda non-polymeric matrix material, such as a ceramic or an oxide glass canbe prepared in the following manner. The solid acid is synthesized formaqueous solution and the matrix material is synthesized separately. Thetwo components are mixed and ground together. The mixture is then hotpressed, preferably at a temperature which causes the solid acid to meltand flow, to yield a dense composite membrane.

[0080] The nature of the chemical bonding in solid acids of generalformula M_(a)H_(b)(XO₄)_(c).nH₂O or M_(a)H_(b)(XO₃)_(c).nH₂O where:

[0081] M is one or more of the species in the group consisting of Li,Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, T₁ and NH₄ ⁺;

[0082] X is one or more of the species in the group consisting of Si, P,S, As, Se, and Te; and

[0083] a, b, c, and n are rational numbers, and n can be zero.

[0084] leads to materials which are inherently poor conductors ofelectrons. These compounds can be used in devices which require bothproton and electron transport directly through the membrane if amechanism for electron transport is introduced.

[0085] The first approach for introducing electronic conductivity intosolid acid based materials is to prepare a composite comprised of thesolid acid and a second substance which has a high electronicconductivity. This second substance may be an electronically conductingpolymer, such as poly(aniline), or a typical metal, such as aluminum orcopper. Where the electronically conducting component is a metal, it maybe advantageous to introduce a chemically and electrically inert polymerinto the composite simply to serve as a binder and provide the membranewith good mechanical properties. The processing methods described abovemay be used to prepare such composite membranes.

[0086] The second approach for introducing electronic conductivity intosolid acid based materials is to perform direct chemical substitutionswith variable valence ions. For example, a portion of the sulfur inCsHSO₄ may be replaced by chromium, which can be present in an oxidationstate of anywhere from 2+ to 6+. Similarly, manganese may be introducedon the sulfur site, as this ion exhibits valence states anywhere between2+ and 7+. Chemical substitution may also be performed with respect tothe cesium in a compound such as CsHSO₄. Large ions with variablevalence, such as thallium, indium, lead and tin can be used for thesesubstitutions. The solid acid so modified may be used in anelectrochemical device directly, or may be combined with a supportingmatrix material as described above.

[0087] In the FIG. 1 embodiment, a membrane-electrode assembly (MEA) isprepared from the CHS-PVDF composite film in which the solid acid topolymer volume ratio is 50:50. The electrodes are formed of graphitepaper which is impregnated with a complex slurry of platinum powder,PVDF, the solid acid, and Nafion, suspended/dissolved in a water andisopropanol solution. After evaporation of the solvents, the electrodesso prepared are hot-pressed onto the composite membrane. The MEA isplaced in a fuel cell test station at 140° C. and hydrogen is introducedat the anode and oxygen at the cathode. The open cell voltage (OCV)obtained in this manner was 0.88 V. The same type of MEA may also beused in the FIG. 2 embodiment.

EXAMPLES Example 1

[0088] A Cs based solid acid such as CsHSO₄, CsHSeO₄ or Cs₅H₃(SO₄)₄.xH₂Ois ground and mixed with a melt-processable polymer binder, such aspoly(vinylidene fluoride), and hot-pressed. The result forms a solidcomposite membrane which is proton conducting even in dry atmospheres.The composite membrane, being comprised of two components whicha resubstantially impermeable to fluids, may be less permeable than Nafion™.

Example 2

[0089] A Cs based solid acid such asCs₃(HSO₄)₂(H_(1.5)(S_(0.5)P_(0.5))O₄), Cs₃(HSO₄)₂(H₂PO₄),Cs₅(HSO₄)₃(H₂PO₄)₂ or Cs₂(HSO₄)(H₂PO₄) is ground and mixed with amelt-processable polymer binder, such as poly(vinylidene fluoride), andhot-pressed. The result forms a solid composite membrane which is protonconducting even in dry atmospheres. The membrane is also less permeableto fluids than Nafion™.

Example 3.

[0090] A NH₄ based solid acid such as (NH₄)₃H(SO₄)₂ or (NH₄)₃H(Seo₄)₂ isground and mixed with a melt-processable polymer binder, such as Crystar101 thermoplastic, and hot-pressed. The result forms a solid compositemembrane which is proton conducting even in dry atmospheres. Themembrane is less permeable to fluids than Nafion™ and is also lessexpensive.

Example 4.

[0091] An solid acid silicate of general formula M_(a)H_(b)SiO₄, such asCaNaHSiO₄, Cs₃HSiO₄, (NH₄)₃HSiO₄, is used as a membrane. Some of thesematerials are water insoluble and may have sufficient structuralintegrity that a binder is not required in some applications.

Example 5.

[0092] A Cs or NH₄ based solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),Cs₅H₃(SO₄)₄.xH₂O or (NH₄)₃H(SO₄)₂ is mixed with the prepolymer of aresin such as “castoglas”, a commercial product from Buehler, Inc. Thepolymerization/crosslinking catalyst is added to the mixture, and asolid composite membrane so formed. The in situpolymerization/crosslinking can lead to a higher impermeability thancomposites formed by melt-processing.

Example 6.

[0093] A Cs or NH₄ based solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),Cs₅H₃(SO₄)₄.xH₂O or (NH₄)₃H(SO₄)₂ is mixed with a monomer such asdicyclopentadiene. A polymerization catalyst is then added to themixture, and a solid composite membrane comprised of the solid acid andpoly(dicyclopentadiene) is formed. The in situ polymerization of thepolymer can lead to a higher impermeability than composites formed bymelt-processing. Use of a NH₄ based solid acid can result in aninexpensive membrane.

Example 7.

[0094] A Cs or NH₄ based solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),Cs₅H₃(SO₄)₄.xH₂O or (NH₄)₃H (So₄)₂ is dissolved in water, and added to asuspension of an insoluble polymer such as poly(vinylidene fluoride)suspended in a fluid such as ethanol. The mixture is cast and theliquids (water and ethanol) allowed to evaporate. This procedure yieldsa composite membrane which is proton conducting even in dry atmospheres.The casting step can produce very thin membranes, with thicknesses onthe order of one hundred microns.

Example 8.

[0095] A Cs or NH₄ based solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),Cs₅H₃(SO₄)₄.xH₂O or (NH₄)₃H (SO₄)₂ is ground and mixed with a ceramic,such as Al₂O₃, or an oxide glass, such as amorphous SiO₂. The mixedpowders are compressed by hot-pressing. The resulting composite membranemay be stable to higher temperatures than those in which the binder is apolymer.

Example 9.

[0096] A Cs or NH₄ based solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),Cs₅H₃(SO₄)₄.xH₂O or (NH₄)₃H(SO₄)₂ is dissolved in water. The solution isintroduced into a porous membrane comprised of an inert binder such asTeflon™, SiO₂, or Al₂O₃. The water is allowed to evaporate, leaving thesolid acid to fill the pores of the binder. The result is a compositemembrane which is proton conducting even in dry atmospheres.

Example 10.

[0097] A Cs or NH₄ based solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),Cs₅H₃(SO₄)₄.xH₂O or (NH₄)₃H(SO₄)₂, which is only proton conducting, isground and mixed with an electronically conducting polymer such aspoly(anylene). The composite membrane formed can conduct both protonsand electrons.

Example 11.

[0098] An solid acid silicate of general formula M_(a)H_(b)SiO₄, such asCaNaHSiO₄, Cs₃HSiO₄ or (NH₄)₃HSiO₄, is ground and mixed with anelectronically conducting polymer such as poly(anilene). The compositemembrane formed can conduct both protons and electrons.

Example 12.

[0099] A proton conducting solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),(NH₄)₃H(SO₄)₂ or CaNaHSiO₄, and a metal, such as Ag, Au, or Cu, areground and mixed. The mixed powders are compressed by hot-pressing. Thecomposite membrane formed can conduct both protons and electrons, andmay be stable to higher temperatures than a composite in which theelectron conducting component is a polymer.

Example 13.

[0100] A proton conducting solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),(NH₄)₃H(SO₄)₂ or CaNaHSiO₄, and a metal, such as Ag, Au, or Cu, areground and mixed. A polymeric material is also added. A solid compositemembrane is prepared either by hot-pressing, if the polymer ismelt-processable such as poly(vinylidene fluoride), or by in situpolymerization, if the polymer is in situ polymerizable such aspoly(dicyclopentadiene). The composite membrane is both proton andelectron conducting, and may have superior mechanical properties to acomposite containing only a solid acid and a metal.

Example 14.

[0101] A mixed electron and proton conducting solid acid, such asCsHCr_(x)S_(1-x)O₄ or (NH₄)₃H(Cr_(x)S_(1-x)O₄)₂ in which one of the Xelements has a variable valence, is mixed with an inert polymericbinder. If the polymer is melt-processable, such as poly(vinylidenefluoride), a membrane is formed by hot-pressing. If the polymer can bepolymerized in situ, a membrane is formed by mixing the solid acid, themonomer and the polymerization catalyst. The resulting membrane conductsboth protons and electrons, and may be more stable in oxidizingatmospheres than a composite containing metal particles.

Example 15.

[0102] A Cs or NH₄ based solid acid, such as CsHSO₄, Cs₂(HSO₄)(H₂PO₄),CsH₃(SO₄)₄.xH₂O or (NH₄)₃H(SO₄)₂ is prepared from aqueous solution,ground, and then pressed into a thin membrane. The membrane is used inan electrochemical device at a temperature above the superprotonictransition temperature and above 100° C., so that the protonconductivity of the solid acid is high and any H₂O that may be presentin the device exists in the form of steam rather than liquid water.

Example 16.

[0103] A mixed electron and proton conducting solid acid, such asCsHCr_(x)S_(1-x)O₄ or (NH₄)₃H (Cr_(x)S_(1-x)O₄)₂ in which one of the Xelements has a variable valence, is prepared from aqueous solution or bysolid state reaction. The powder is then ground and pressed into a thinmembrane. The membrane is used in an electrochemical device at atemperature above the superprotonic transition temperature and above100° C., so that the conductivity of the solid acid is high and any H₂Othat may be present in the device exists in the form of steam ratherthan liquid water.

Example 17.

[0104] A composite comprised of one or more of the solid acids listed inTable 1 and one or more of inert binders listed in Table 2. If one ormore of the components in the composite is electronically conducting,the composite membrane will be capable of conducting both protons andelectrons. Electronically conducting substances are indicated. TABLE 1Solid acid compounds. Sulfates and selenates and sulfate-phosphatesselenate phosphates silicates CsHSO₄ CsHSeO₄ CaNaHSiO₄ Cs₃H (SO₄) ₂ Cs₃H(SeO₄) ₂ CaH₂SiO₄ Cs₅H₃ (SO₄) ₄.xH₂O Cs₅H₃ (SeO₄) ₄ .xH₂O CsH₃SiO₄ Cs₃(HSO₄) ₂ (H_(1.5) (S_(0.5)P_(0.5) Cs₃ (HSeO₄) ₂ (H_(1.5) (Se_(0.5)Cs₂H₂SiO₄ )O₄) P_(0.5)) O₄) Cs₃ (HSO₄) ₂ (H₂PO₄) Cs₃ (HSeO₄) ₂ (H₂PO₄)Cs₃HSiO₄ Cs₂ (HSO₄) (H₂PO₄) Cs₂ (HSeO₄) (H₂PO₄) NH₄H₃SiO₄ Cs₅ (HSO₄) ₃(H₂PO₄) ₂ Cs₅ (HSeO₄) ₃ (H₂PO₄) ₂ (NH₄) ₂H₂SiO₄ CsH₂PO₄ (NH₄) ₃HSiO₄NH₄HSO₄ NH₄HSeO₄ RbH₃SiO₄ (NH₄) ₃H (SO₄) ₂ (NH₄) ₃H (SeO₄) ₂ Rb₂H₂SiO₄(NH₄) ₅H₃ (SO₄) ₄ .xH₂O (NH₄) ₅H₃ (SeO₄) ₄ .xH₂O Rb₃HSiO₄ (NH₄) ₂ (HSO₄)(H₂PO₄) (NH₄) ₂ (HSeO₄) (H₂PO₄) KH₃SiO₄ (NH₄) H₂PO₄ K₂H₂SiO₄ RbHSO₄RbHSeO₄ K₃HSiO₄ Rb₃H (SO₄) ₂ Rb₃H (SeO₄) ₂ NaH₃SiO₄ Rb₅H₃ (SO₄) ₄ .xH₂ORb₅H₃ (SeO₄) ₄ .xH₂O Na₂H₂SiO₄ Rb₂ (HSO₄) (H₂PO₄) Rb₂ (HSeO₄) (H₂PO₄)Na₃HSiO₄ RbH₂PO₄ BaCsHSiO₄

[0105] TABLE 2 Binder or matrix materials ceramic/oxide metal or Polymerglass semiconductor poly (vinylidene fluoride) SiO₂ Ag* poly(dicyclopentadiene) Al₂O₃ Au* poly (tetraflouroethelyne) MgO Cu*[Teflon] poly (ether-ether ketone) cordierite Al* poly (ether sulfone)Ni* Silicones [dimethyl Fe* siloxane polymers] poly (pyrrole)* Zn* poly(aniline)* graphite* silicon*

[0106] Other modifications are within the disclosed embodiment. Forexample, the above has described the materials having a superprotonictransition upon heating. Certain materials may have their superprotonictransition temperature below room temperature. Thus, there may be noapparent superprotonic transition and the material would be disorderedat room temperature. These solid acids with structural disorder evenprior to heating are also contemplated.

What is claimed is:
 1. A proton conducting membrane, formed of a solidacid material includes the chemical form M_(a)H_(b)(XO_(t))_(c), andwhich is of a type that transports H+ ions, where a, b, c and t arenon-zero integers in a solid phase, said membrane having a structurethat can be used in an aqueous environment.
 2. A proton conductingmembrane, formed of a solid acid material which includes the chemicalform M_(a)H_(b)(XO_(t))_(c), and which is of a type that is capable of asuperprotonic transition where a, b, c and t are non-zero integers in asolid phase, said membrane having a structure that can be used in anaqueous environment.
 3. A membrane as in claim 1 wherein t is 3 or
 4. 4.A membrane as in claim 1 wherein said solid acid material is of thegeneral form Cs_(a)H_(b)(XO_(t))_(c), where a, b, c, and t are non-zerointegers.
 5. A membrane as in claim 1 where X is silicon.
 6. A membraneas in claim 3 wherein M is Cs.
 7. A membrane as in claim 3 wherein M isNH₄.
 8. A membrane as in claim 3 wherein said solid acid is of the formM_(a)H_(b)(XO_(t))_(c).nH₂O, where a, b, c, and t are positive non-zerointegers, and n is an integer.
 9. A membrane as in claim 4 wherein X isP.
 10. A membrane as in claim 1, further comprising an electrochemicaldevice, using said membrane for proton transport.
 11. A protonconducting membrane, formed of a solid acid material in a superprotonicphase, said solid acid material being of the general formulaM_(a)H_(b)(XO_(t))_(c), where t is 3 or 4, and having a structurewhereby oxyanions are linked together by hydrogen bonds, where the Mmaterial is at least one material from the group consisting of Li, Be,Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Tl or NH₄ ⁺, and the X material is atleast one material from the group consisting of Si, P, S, As, Se, or Teand a, b, and c are non-zero integers, and wherein said protonconducting membrane is used at temperatures greater than 100° C.
 12. Aproton conducting membrane, comprising: a water-soluble solid acidmaterial which includes the chemical form M_(a)H_(b)(XO_(t))_(c), wherea, b, c and t are non-zero integers and which is used in a structuralbinder for said solid acid material, forming a membrane with said solidacid material.
 13. A membrane as in claim 12 wherein said structuralbinder is a polymer.
 14. A membrane as in claim 13 wherein said solidacid material is a type capable of a superprotonic transition at aspecified temperature.
 15. A membrane as in claim 13 wherein saidpolymer is a melt processable polymer.
 16. A membrane as in claim 13wherein said polymer is an in-situ polymerized polymer.
 17. A membraneas in claim 12 wherein said structural binder is a ceramic.
 18. Amembrane as in claim 12 wherein said structural binder includes silicon.19. A membrane as in claim 12 wherein said solid acid is of the generalformula M_(a)H_(b)(XO_(t))_(c), where: the M material is a material fromthe group consisting of Li, Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Te or NH₄⁺, and the X material is from the group consisting of Si, P, S, As, Se,or Te and where a, b, c, and t are non-zero integers.
 20. A protonconducting membrane, formed of a solid acid material in a superprotonicphase usable in an aqueous environment over 100° C.